Is the Person Giving you Power of Attorney to Act for Him or Her Mentally Capable?
In order for a power of attorney to be effective, the person granting you power of attorney would have to be mentally competent at the time he or she grants power of attorney. If the person who is granting you power of attorney is not okay with authorizing you to act right from the get go, then the trip of power of attorney granted should be a "springing" power of attorney that in effect springs into life upon the person who granted the power becoming incapable of acting for him or herself.
Further, in order for most powers of attorney to remain effective in the event the grantor of the power should no longer be mentally competent, it must be a "durable power of attorney" otherwise (in most states) the power of attorney would become ineffective at any time the grantor no longer is capable -- which is when the person probably wanted you to be able to act for him or her, without having to go to court to appoint an conservator or special guardian.
One more suggestion -- if the person wants to give you power of attorney so that you can access his or her bank account, so you would be able to pay his or her bills on time, it is usually a good idea to go to the bank in advance and ask for its form. Even though they are not supposed to, banks and similar financial institutions often give those named as attorneys in fact for their depositors a very hard time unless the form of power of attorney satisfies the bank's lawyer. So that often means it makes sense to get both a general power of attorney AND each bank's form of power of attorney signed. Also make sure the grantor's signature is notarized on each power of attorney form.